Frost can spread not only along surfaces but also via suspended “ice bridges” that form above them. The discovery of this previously unknown frost-propagation pathway could lead to new ways of making surfaces that resist frost growth, thereby improving the performance of devices that operate in cold, humid environments.
Frost accumulation is a major problem in refrigerators, aeroplanes and heat pumps, to name just a few. On the microscale, it primarily spreads from one individual freezing water droplet to the next via two-dimensional bridges, or causeways, that temporarily form on the surface of an object. Although the wettability of the surface is known to affect the spreading process, the mechanism behind this effect was poorly understood.
Ice bridges can grow in two distinct spatial modes
To learn more, a team led by physicist Nenad Miljkovic at the University of Illinois Urbana-Champaign, US imaged the channel-forming process using high-speed high-resolution optical microscopy combined with a profilometry technique called focal plane shift imaging (FPSI). They found that frost can spread in two distinct ways. On hydrophilic surfaces, causeways form along the substrate, in line with current theoretical models. On superhydrophobic surfaces, however, the situation is quite different. Here, frost spreads via ice bridges that are suspended above the surface in three-dimensional space.
This suspended or “out-of-plane” growth mode represents a fundamentally different pathway for frost propagation, says team member Siyan Yang, the first author of a Nature Physics paper about the work. Previous studies likely overlooked this mechanism, Yang adds, because of limitations in experimental observations.
Superhydrophobic coatings nearly double the frost propagation time
The researchers also studied the growth rate of the different bridge types. They found that suspended bridges grew slower than bridges on the surface due to the reduced thermal coupling between the bridges and the cold substrate. This reduced coupling correspondingly reduces the vapour pressure difference between ice and water droplets (which depends on surface droplet geometry, itself controlled by wettability) and drives down ice growth. Indeed, the team found that the speed at which frost spreads fell more than 80% in this mode.
To test the practical relevance of their findings, the researchers applied superhydrophobic coatings to metre-sized structures such as the finned-tube aluminium heat exchangers commonly found in air conditioners, refrigerators and automotive systems. Condensation frosting on such systems poses a major efficiency challenge because frost has an inherently low thermal conductivity. When it accumulates on heat exchangers, it therefore severely impedes heat being exchanged with the surrounding air.
On uncoated commercial heat exchangers that are inherently hydrophilic, the team found that frost rapidly forms and spreads across the fins. “In the superhydrophobic counterparts, however, the onset of frost formation is delayed, and it propagates much more slowly,” Yang says. In the two kinds of commercial systems they tested, she adds, applying superhydrophobic coatings nearly doubled the frost propagation time.
Humidity controls frost pattern formation
The results suggest that designers of anti-frost surfaces could benefit from trying this new strategy, Yang says. “Rather than focusing solely on delaying initial ice nucleation, surfaces could be engineered to control the geometry of ice-bridge growth and interrupt frost spreading, thereby improving the performance and energy efficiency of a host of equipment operating in cold and humid environments,” she tells Physics World.
The team is now investigating how surface chemistry and surface structures influence suspended ice-bridge formation and frost propagation. “We are also exploring ways to translate the fundamental mechanism into scalable anti-frost coatings and heat-exchanger technologies,” Yang reveals. “Ultimately, our goal is to establish predictive design rules that connect microscale ice-bridge dynamics with real-world frost management performance.”
